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Strangeness Enhancement in p+p, p+Pb and Pb+PbCollisions at LHC
EnergiesYuuka Kanakubo1, Michito Okai1, Yasuki Tachibana2 and
Tetsufumi Hirano1
1Department of Physics, Sophia University, 7-1 Kioi-cho,
Chiyoda-ku, Tokyo 102-8554, Japan2 Department of Physics and
Astronomy, Wayne State University, 666 W. Hancock St., Detroit,
MI48201, USA
E-mail: [email protected]
(Received April 4, 2019)
Recently ALICE Collaboration reported enhancement of yield ratio
of multi-strange hadrons tocharged pions as a function of
multiplicity at mid-rapidity in proton–proton (p+p),
proton–lead(p+Pb) and lead–lead (Pb+Pb) collisions at the LHC.
Motivated by these results, we have developedthe dynamical
core–corona initialization framework which enables us to describe
p+p and p+Pb col-lisions as well as Pb+Pb, and we investigate
whether the quark gluon plasma (QGP) is created insmall colliding
systems by analyzing various hadron yields and their ratios
systematically. We findthat our results reproduce tendencies of the
ALICE data especially for multi-strange hadrons. Theseresults
indicate that the QGP is partly formed in high multiplicity events
in small colliding systems.
KEYWORDS: Quark gluon plasma, Relativistic heavy-ion collisions,
Small colliding systems,Core–corona picture, Strangeness
enhancement
1. Introduction
High energy heavy-ion collision experiments are performed at the
Relativistic Heavy Ion Col-lider (RHIC) in Brookhaven National
Laboratory and the Large Hadron Collider (LHC) in CERN tounderstand
properties of the quark gluon plasma (QGP). It is known that
various experimental dataare described by relativistic
hydrodynamics, which indicates that the QGP behaves nearly like a
per-fect fluid. Conventionally, it has been assumed that the QGP is
generated only in heavy-ion collisionsand that small systems such
as proton-proton or proton–nucleus collisions provide references
for ex-tracting medium effects in heavy-ion collisions. Recently,
ALICE Collaboration obtained surprisingresults which indicate,
however, the QGP formation in small colliding systems [1]. They
measuredyield ratios of multi-strange hadrons to charged pions as
functions of multiplicity at mid-rapidity andthe results exhibit
rapid increase with multiplicity in proton–proton (p+p) collisions.
Moreover, theratios do not seem to depend on the system size or
collision energies. One of the possible descriptionto interpret
this result is the core–corona picture [2–6]. The core–corona
picture is a two-componentdescription which is described by
chemically equilibrated matter and unscathed partons. In this
studywe introduce the core–corona picture into the dynamical
initialization model which was proposed inRef. [7]. Under the
core–corona picture, initially produced partons tend to become
fluids in denseregion in which a lot of interactions among partons
are assumed to happen, while partons do not tendto become fluids in
dilute region in which few interactions occur. We introduce the
above picture intothe fluidization rate in the dynamical
initialization model [8] and analyze the multiplicity dependenceof
particle yield ratios in various colliding systems.
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2. Model
Firstly, we mention our model flow briefly. In this framework
the QGP fluids are generated frompartons produced initially just
after the first contact of collisions. First we generate the
initial par-tons using a Monte Carlo event generator PYTHIA ver
8.230 [9, 10], with the hadronization optionswitched off in order
to define phase space distributions of the partons. Next we perform
the dynami-cal core–corona initialization to obtain the initial
condition of the QGP fluids. After the initialization,we solve
ideal hydrodynamic equations with source terms in fully
(3+1)-dimensional space as usual.We start initialization of the QGP
fluids from τ00 = 0.1 fm which is assumed to be formation time
ofthe partons and continue until τ0 = 0.6 fm which is initial time
of the fluids. After the hydrodynamicsimulations, we calculate
final hadron yields from the core and the corona separately. We
obtain finalhadron yields from the core integrating the Cooper–Frye
formula [11] at chemical freeze-out surface.We consider the
corrections of yields from resonance decays by multiplying factors
estimated from astatistical model [12]. On the other hand, we
calculate final hadron yields from the corona perform-ing string
fragmentation using PYTHIA. Thus, the final hadron yield in this
framework is the sum ofthese two final yields from both the core
and the corona.
The source term in the hydrodynamic equation can be defined
as
Jµ(x) = −∑
i
dpµidt
G(x − xi(t)), (1)
where pµi is the four momentum of the i th parton obtained from
PYTHIA and the summation is takenover all partons in the event.
Here we employ the Gaussian function, G, for smearing energy
andmomentum deposited at the position of the parton. To take
account of the core–corona picture, weparametrize the rate of
energy and momentum deposition of the parton as
dpµidt
(t) = −a0ρi(xi(t))pT,i2(t)
pµi (t), (2)
ρi(x) =∑j,i
G(x − x j(t)). (3)
Here, a0, ρi and pT,i are control parameter for magnitude,
density of partons surrounding the i th par-ton and transverse
momentum. Under this formulation, partons in dense region are
likely to becomefluids while those in dilute region tend to
survive.
3. Results
Collision systems and energies in these simulations are p+p,
p+Pb and Pb+Pb collisions at√sNN = 7, 5.02 and 2.76 TeV,
respectively, and Au+Au collisions at
√sNN = 200 GeV. Figure 1
shows the particle yield ratio in those collision systems as a
function of multiplicity from our frame-work compared with the
experimental data [1,13–18]. For (a) cascades (Ξ−+Ξ̄+), (b) lambdas
(Λ+Λ̄)and (c) phi mesons (φ), our results show good agreement with
the experimental data. The ratios in-crease up to 〈dNch/dη〉 ∼ 100
and saturate in high multiplicity events. In Fig. 1(a), we also
plotthe yield ratios from string fragmentation and that from fluids
as references. Since our result in lowmultiplicity p+p events is
almost identical with the one from string fragmentation,
contribution fromstring fragmentation turns out to be dominant. On
the other hand, the yield ratio of full calculationincreases with
multiplicity towards the one from fluids. This tendency implies
that the contribution offluids becomes larger and dominant at high
multiplicity events. Moreover, our result seems to behave
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just as a function of multiplicity, and are independent of their
system size or collision energies. For(d) proton (p + p̄) yield
ratio, a deviation between our result and the experimental data is
seen above〈dNch/dη〉 ∼ 50. This would be because proton and
anti-proton annihilation could happen in the latehadronic
rescattering stage. We leave consideration of this effect for the
future work.
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